Many types of printers are known and they include ink jet, laser and various thermal and impact printers. Ink jet printers include those that are thermally actuated (e.g., resistive element) and those that are mechanically actuated (e.g., piezo-electric element). Representative ink jet printers include those made by Hewlett Packard, Canon and Epson, etc. The electromagnetic interference (EMI) reducing techniques of the present invention are applicable to all printers and particularly to ink jet printers.
Advances in semiconductor fabrication and printhead design have led to an increase in the number of firing chambers or drop generators provided on a printhead. In a representative prior art printhead each of the plurality of firing chambers or subset thereof, may be fired simultaneously.
An increase in the number of firing chambers on a printhead leads to an increase in printed image resolution and may result in improvements in image quality and the rate at which an image (or document) is printed.
While the ability to fire multiple printheads simultaneously is advantageous in delivering ink to a desired destination (e.g., a sheet of paper), multiple simultaneous firings are disadvantageous in that they generate a significant amount of EMI due to the multiple simultaneous firing signal transitions. In other words, the firing signal for each firing chamber may change from an off state to a drive state simultaneously (i.e., large current change (.DELTA.i) in a small time change (.DELTA.t)), causing the firing signal conductors to function as de-facto antenna that radiate electromagnetic interference generated by the abrupt signal transitions. Excess EMI causes interference with or the failure of system components and impedes receiving approval from the FCC and like international agencies that set EMI emission standards.
This problem is exacerbated by continuing efforts to increase firing chamber densities. Not only do higher density circuits have more EMI generation points, but they are also more likely to be adversely affected by the deleterious effects of EMI.
Current attempts to reduce or minimize the effects of EMI have relied primarily on shielding. This may take the form of shielded cables, grounded conductive coatings on the inside of plastic printer housings, ferrite beads placed around conductors and providing EMI generating and conducting components in a grounded sheet metal box or the like. These steps add significant expense to the cost of printers and complicate manufacture.
Another disadvantageous aspect of conventional printers is that simultaneous firing of multiple firing chambers results in a significant instantaneous draw on the power supply, resulting in the use of more expensive and larger power supplies and more frequent power supply failure.